The present invention relates to structured packing. The structured packing has particular application in exchange columns, especially in cryogenic air separation processes, although it also may be used in other applications, such as heat exchangers.
The term, "column", as used herein, means a distillation or fractionation column or zone, i.e., a column or zone wherein liquid and vapor phases are countercurrently contacted to effect separation of a fluid mixture, such as by contacting of the vapor and liquid phases on packing elements or on a series of vertically-spaced trays or plates mounted within the column.
The term "packing" means solid or hollow bodies of predetermined size, shape, and configuration used as column internals to provide surface area for the liquid to allow mass transfer at the liquid-vapor interface during countercurrent flow of two phases. Two broad classes of packings are "random" and "structured".
"Random packing" means packing wherein individual members do not have any particular orientation relative to each other or to the column axis. Random packings are small, hollow structures with large surface area per unit volume that are loaded at random into a column.
"Structured packing" means packing wherein individual members have specific orientation relative to each other and to the column axis. Structured packings usually are made of expanded metal or woven wire screen stacked in layers or as spiral windings.
In processes such as distillation or direct contact cooling, it is advantageous to use structured packing to promote heat and mass transfer between counterflowing liquid and vapor streams. Structured packing, when compared with random packing or trays, offers the benefits of higher efficiency for heat and mass transfer with lower pressure drop. It also has more predictable performance than random packing.
Cryogenic separation of air is carried out by passing liquid and vapor in countercurrent contact through a distillation column. A vapor phase of the mixture ascends with an ever increasing concentration of the more volatile components (e.g., nitrogen) while a liquid phase of the mixture descends with an ever increasing concentration of the less volatile components (e.g., oxygen). Various packings or trays may be used to bring the liquid and gaseous phases of the mixture into contact to accomplish mass transfer between the phases.
There are many processes for the separation of air by cryogenic distillation into its components (i.e., nitrogen, oxygen, argon, etc.). A typical cryogenic air separation unit 10 is shown schematically in FIG. 1. High pressure feed air 1 is fed into the base of a high pressure distillation column 2. Within the high pressure column 2, the air is separated into nitrogen-enriched vapor and oxygen-enriched liquid. The oxygen-enriched liquid 3 is fed from the high pressure distillation column 2 into a low pressure distillation column 4. Nitrogen-enriched vapor 5 is passed into a condenser 6 where it is condensed to provide reboil to the low pressure column 4. The nitrogen-enriched liquid 7 is partly tapped 8 and is partly fed 9 into the low pressure column 4 as liquid reflux. In the low pressure column 4, the feeds (3,9) are separated by cryogenic distillation into oxygen-rich and nitrogen-rich components. Structured packing 20 may be used to bring into contact the liquid and gaseous phases of the oxygen and nitrogen to be separated. The nitrogen-rich component is removed as a vapor 11. The oxygen-rich component is removed as a vapor 12. Alternatively, the oxygen-rich component can be removed from a location in the sump surrounding reboiler/condenser 6 as a liquid. A waste stream 13 also is removed from the low pressure distillation column 4. The low pressure distillation column 4 can be divided into multiple sections. Three such sections (4A, 4B, 4C) are shown in FIG. 1 by way of example.
The most commonly used structured packing consists of corrugated sheets of metal or plastic foils or corrugated mesh cloths stacked vertically. These foils may have various forms of apertures and/or surface roughening features aimed at improving the heat and mass transfer efficiency. However, the flow of both liquid and vapor is largely confined to the space between the sheets. Such packing lacks symmetry and thus the flow characteristics are highly non-isotropic. Liquid and vapor introduced between a pair of sheets tend to stay confined between that pair of sheets. A solution to this is to rotate successive layers of the structured packing sheets, typically by an angle of 90.degree.. An example of such packing is disclosed in U.S. Pat. No. 4,296,050 (Meier). However, each rotation increases the pressure drop through the column comprised of the packing.
Attempts have been made in the past to produce a packing which leads to more isotropic fluid flow through a column including such packing.
For example, U.S. Pat. No. 4,830,792 (Wilhelm) discloses the use of horizontally superposed layers of packing, each of which is provided with adjacent pyramidal formations. Alternating fan-blade like structures are thereby formed which impart a vortex motion to ascending vapor in both clockwise and anti-clockwise directions. U.S. Pat. No. 5,158,712 (Wilhelm) and U.S. Pat. No. 5,500,160 (Suess) disclose similar layers of packing. However, since these prior art layers of packing are limited in the way that they can be stacked, the configurations and symmetry which can be provided by stacking these prior art layers of packing are limited.
It also is well-known in the prior art that mesh type packing helps spread liquid efficiently and gives good mass transfer performance, but mesh type packing is much more expensive than most foil type packing.
It is desired to have a specific structured packing that shows high performance characteristics for cryogenic applications, such as those used in air separation, and for other heat and mass transfer applications.